From your log file I see that Postfix tries to connect to MySQL on houston2.ustabilserverplads.dk instead of 127.0.0.1. So either you allow remote connections to that MySQL user, or you change the configuration so that Postfix tries to connect on 127.0.0.1.

Have a look in the table mysql.user and mysql.db. Is the host field of the user mail_admin set to "localhost" or "%"? Have you restarted the mysql server after changing the permissions and adding the mail_admin user?

Have a look in the table mysql.user and mysql.db. Is the host field of the user mail_admin set to "localhost" or "%"? Have you restarted the mysql server after changing the permissions and adding the mail_admin user?